Basically, this industry refers to recovery of the animal by-products produced in slaughter houses or meat packing plants where all waste animal tissue or by-products are processed to obtain tallow (animal fat) and meat & bone meal as final products. The rendering process includes grinding and drying the material in batch type or continuous cookers and separates the fat from the bone and protein. A rendering process yields a fat commodity (yellow grease, choice white grease, bleachable fancy tallow, etc.) and a protein meal (meat and bone meal, poultry byproduct meal, etc.). This process is divided into 2 groups, Edible rendering processes are basically meat processing operations and produce lard or edible tallow for use in food products. Edible rendering is generally carried out in a continuous process at low temperature (less than the boiling point of water). The process usually consists of finely chopping the edible fat materials (generally fat trimmings from meat cuts), heating them with or without added steam, and then carrying out two or more stages of centrifugal separation. The first stage separates the liquid water and fat mixture from the solids. The second stage further separates the fat from the water. The solids may be used in food products, pet foods, etc., depending on the original materials. The separated fat may be used in food products, or if in surplus, it may be diverted to soap making operations.
Most edible rendering is done by meat packing or processing companies. On the other hand, materials that for aesthetic or sanitary reasons are not suitable for human food are the feed stocks for Inedible Rendering Processes. Much of the inedible raw material is rendered using the “dry” method. This may be a batch or a continuous process in which the material is heated in a steam-jacketed vessel to drive off the moisture and simultaneously release the fat from the fat cells. The material is first ground, then heated to release the fat and drive off the moisture, percolated to drain off the free fat, and then more fat is pressed out of the solids, which at this stage are called “cracklings” or “dry-rendered tankage”. The cracklings next go to expeller presses to extract and separate additional tallow and the meal is ground to make meat and bone meal. A variation on a dry process involves finely chopping the material, fluidizing it with hot fat, and then evaporating the mixture in one or more evaporator stages. Some inedible rendering is done using a wet process, which is generally a continuous process similar in some ways to that used for edible materials. The material is heated with added steam and then pressed to remove a water-fat mixture which is then separated into fat, water and fine solids by stages of centrifuging and/or evaporation. The solids from the press are dried and then ground into meat and bone meal. Most independent renderers process only inedible material. Rendering cookers (Batch or Continuous) and Expeller Presses are the heart of this processes since they are the two pieces of equipment were the separation takes the fat from the solids portion of raw materials.